The invention relates to a particulate filtration device.
Particulate filters are becoming more and more a standard part in exhaust after treatment for diesel engines. The most commonly used particulate filters for heavy duty engines are based on ceramic substrates, such as cordierite or silicon carbide (SiC). They are usually made as monolith substrates, with a large number of parallel, single ended channels where a part of the channels is closed at the inlet of the monolith, and part of the channels are closed at the outlet of the monolith. The particulates are filtered when the exhaust gas is forced through the channel walls of the substrate. The efficiency is usually high, above 90%. The collected particulates are removed from the filter by oxidation. The oxidation may occur at lower temperatures starting at about 250° C. if NO2 is present in the exhaust gases, or by oxygen at higher temperatures of about 600° C. The temperature control during high temperature oxidations is critical for the durability of the particulate filter. The particulate filter is especially sensitive to local temperature maxima. Such local temperature maxima can be caused by irregular soot collection or by imperfect regeneration procedures of the filter. Some of the collected soot cannot be removed by oxidation, and the ashes left after the regeneration must be handled separately, mainly manually by demounting the muffler and washing the filter.
Filtration devices described above have been commercially available for over 20 years. In U.S. Pat. No. 4,589,983 A several ways of putting together corrugated and flat filter materials are shown. It is proposed to stack corrugated filter sheets where fluid channels of successive sheets are oriented orthogonally to each other with a 90° rotation. A feed gas enters at a feed gas side and a filtrate, e.g. filtered exhaust gas if raw exhaust gas is the feed gas, exits the stack at sides orthogonal to the feed gas side.
In U.S. Pat. No. 6,126,833 A an arrangement is disclosed with small ceramic monoliths to get a cross flow filtration where the filtrate can move to the outside of the module.
An example of stacking sintered metal sheets to a particulate filter is given in EP 1 132 582 A1. Large sheets with a central opening are welded together in an accordion like style, where exhaust gas is directed from the outside through the porous sheets into the inside opening. The retentate, i.e. soot and ashes, is collected in a container which is arranged below the filter sheets.
It is desirable to provide a particulate filtration device which allows for easy removal of soot and ashes from the filter.
According to an aspect of the invention, a particulate filtration device comprising a filter with a feed inlet side for a feed gas and an filtrate outlet for a filtrate, further comprising feed gas channels and filtrate channels and one or more diaphragms between the feed inlet side and the filtrate outlet. Opposite to the feed inlet side a particulate outlet is arranged with a collector compartment attached to the particulate outlet for collecting particulates retained in the feed gas channels. Favourably, a particulate outlet is arranged opposite to the feed inlet side and the filtrate outlet is arranged crosswise to the feed inlet side. Preferably, the diaphragms between the feed gas and filtrate channels are oriented perpendicular to the stack direction. The filter body can exhibit a cube-like shape. Preferably, the filtrate outlet can be provided at two opposite sides of the filter. The cross-flow arrangement of the filter yields a compact and efficient filtration device.
A collector compartment is arranged at the particulate outlet for collecting particulates retained in the feed gas channels. The collector compartment collects the ashes formed during regeneration of the filter. The filtration device is preferably used in automotive applications, particularly in vehicles equipped with diesel engines. Thus, the compartment can be adapted in its size to hold ashes formed at least in between regular service intervals. Particularly, the volume of the collector compartment can be designed to hold ashes in between several service intervals of the vehicle. The feed gas, i.e. the exhaust of the engine in an automotive ambient, enters the feed gas channels of the filtration device and flows through the gas-permeable diaphragms into the filtration channels and exits the filtration device. Ashes and soot carried in the feed gas is retained by the diaphragms. The soot can be burned by NO2 and/or O2, while the ash is slowly moved by viscous forces to the end of the feed channels into the collector compartment. Favourably, the channels are of considerably size with cross-section areas in the range of several mm2.
Favourably, the collector compartment can be connected releasably to the filter. Removing the ashes out of the filtration device and cleaning the collector compartment can be done in a very simple way.
According to a preferred embodiment, the collector compartment can be mounted geodetically below the filter when the filtration device is mounted in its working ambient. For instance, the filter and the collector compartment attached can be oriented vertically instead of horizontally, with the feed inlet side at the upper side and the collector compartment at the lower side. Additionally to the viscous forces which drives the ash to the end of the feed channels into the collector compartment, in the proposed arrangement this driving force is supported by gravity. Preferably, feed gas is deflected with respect to the feed inlet side, for example by providing a duct which is inclined to the feed inlet side of the filter.
Preferably, the feed gas can move along feed gas channels arranged in feed gas plates and/or the filtrate can move along filtrate channels arranged in filtrate plates. The arrangement is a simple design. Particularly, the channels of successive plates can be aligned orthogonally.
According to an advantageous embodiment the plates are stacked in a stack direction and separated by gas-permeable diaphragms. This results in a compact and simple filter design.
In a further preferred embodiment, a catalytic material can be provided in one or more or all feed gas channels. Preferably, the one or more or all feed gas channels can provide an oxidation catalyst. The catalytic material can be preferably applied as a catalytic layer. By way of example, the catalyst can increase the amount of NO2 thus improving a continuous filter regeneration at relatively low temperatures and/or increasing a NOx removal efficiency. Thereby, oxidation of soot in the feed gas channels can be improved.
Additionally or alternatively, a catalytic material can be provided in the one or more filtrate gas channels. Preferably, one or more or all filtrate channels can provide a catalyst for reducing nitrogen oxides (NOx). An alternative catalyst can be a catalyst which reduces NO2 to NO which can be preferred especially if a high NO2 content is needed for soot oxidation. The catalytic material can be preferably applied as a catalytic layer.
One or more separator walls between gas channels within one plate can be gas tight. In such a case the feed gas can only penetrate the diaphragm between the feed channels and the filtrate channels. A preferred embodiment of the plates is a comb-like cross-section with a multitude of parallel separator walls which are on one side connected with a plate-like porous body which is forming the bottom of the gas channels. The individual plates are open when seen from above with a porous bottom and the parallel separator walls standing upright on the bottom. The bottom formed of porous material serving as diaphragm which separates the filtrate from the retentate when the feed gas flows through the diaphragm. Plates of this type can be fabricated as monolithic bodies. Stacking a plurality of such plates on top of each other yields an arrangement with channels where the bottom of the successive plate forms the cover of the channels of the previous plate. Thus, bottom and cover of a channel are permeable for the feed gas. In this embodiment, the hydraulic diameter of the channels can be relatively large resulting in a lower pressure drop or, alternatively, can have a large diaphragm area per volume at a given hydraulic diameter. The hydraulic diameter is a commonly used term when handling flow in noncircular, tubes and channels. It can be defined as four times the cross sectional area of the channel divided by the fluid-wetted perimeter of the channel. A fluid is defined as a substance that continually deforms (flows) under an applied shear stress regardless if how small the applied stress. All liquids and all gases are fluids.
An alternative embodiment for the plates is to use corrugated foils between diaphragm plates. This embodiment is very advantageous in combination with an oxidation catalyst in the feed gas channels and/or a NOx-reducing catalyst in the filtrate channels, as this structure is less sensitive to pressure drops. The catalyst coating can be arranged on the corrugated foils instead of the diaphragm.
Additionally or alternatively, one or more or all of the separator walls between gas channels within one plate can act as diaphragms. This allows to manufacturing the plates of the filter in one piece out of the same material making the manufacture fast and economical. Additionally, in case the flow is blocked in one or more channels, the fluid can flow through the permeable separator walls from a blocked channel to an open channel resulting in a homogenized fluid flow between the channels in a plate.
Additionally or alternatively, one or more separator walls between gas channels within the same plate can be perforated. A fluid flow in the channels can be redistributed if parts of the filter are suffering from larger pressure drops caused, e.g. by an inhomogeneous distribution of the particulates retained by the diaphragms.
According to an advantageous embodiment, an injection unit can be coupled to the filter in direct fluid connection with the filtrate channels. Preferably, the injection unit can replace one filtrate outlet. The injected medium can be a reducing agent for NOx removal such as ammonia or urea or another ammonia carrier, which is typically used for ammonia-SCR (SCR=selective catalytic reduction). The injection medium can also be fuel used for reducing NOx in a hydrocarbon-SCR process (HC-SCR) or for reducing NO2 partly or completely to NO.
A part of the feed gas favourably can be redirected to the injection unit as driving gas for an injected medium. An appropriate amount of redirected feed gas can be in the range of up to 15 vol. %, preferably up to 10 vol. %. The amount of redirected feed gas can be adjusted to provide enough energy and residence time to allow evaporation and/or decomposition of the injected medium. Preferably, a fluid conduit is provided between the downstream end of the feed gas channels and the injection unit. The amount of redirected feed gas can easily be adapted by the cross-section of the gas conduit attached to the feed gas outlet. The redirected gas can supply energy to the reducing-agent droplets for evaporation and/or decomposition as well as a driving force towards the filter.
The fluid conduit can be arranged juxtaposed to an interface between the filter and the injection unit. This yields an advantageous pre-heating of the injected medium. The injector side of the filter can act as a heat exchanger and cool the feed gas and the filtrate, respectively, thus heating the injected medium. If the temperature of the feed gas is too low to yield a proper evaporation and/or decomposition of the injected reducing agent, a separate heater can be coupled to the arrangement. For example, urea needs about 400° C. as working temperature yielding a reliable decomposition into the desired products. If the heat from the exhaust can be used to preheat the injected medium, less energy is required for heating.
Preferably, a particulate filter can be provided in the fluid conduit. The filter can be of the same type as the filter described above, which is a wall-flow type. The filter can use the same collector compartment for the ash. However, any kind of filter can be used in the fluid conduit, such as a flow-through type etc.
Further, a heater can be coupled to the fluid conduit. The heater can preferentially be an electric heater. Generally, a catalytic burner or a flame burner can also be employed.
In a preferred embodiment, the channel plates and/or the diaphragms can be made of metal. Using metallic materials, particularly sintered metal, the filter exhibits a higher thermal conductivity and a higher heat capacity than a ceramic material, thus preventing thermal inhomogeneity and hot spots. Metallic materials also exhibit better mechanical properties compared to ceramics.
Generally, the diaphragms can also be made of any porous material. Using porous metal, i.e. sintered metal, yields a high thermal conductivity, thus avoiding local thermal spots.
In the drawings, equal or similar elements are referred to by equal reference numerals. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. Moreover, the drawings are intended to depict only typical embodiments of the invention and therefore should not be considered as limiting the scope of the invention.